$g(x) = 2x+9$ $g\Big($
Explanation: We need to solve an equation with the function's formula $2x+9$ on one side and the given output $15$ on the other side. $\begin{aligned} 2x+9&=15\\\\ 2x&=6\\\\ x&=3\end{aligned}$ Therefore, $g(3)=15$.